1. Field of the Invention
The present invention relates to a power semiconductor devices and, more particularly, to a semiconductor device with a new buffer structure (hereinafter referred to as static induction buffer structure) which permits reduction of the resistance of the buffer layer, increases the injection ratio of holes from the anode and allows the application a high-intensity electric field across the cathode and anode. The invention also pertains to a semiconductor device with a new buffer structure (hereinafter referred to as a drift buffer structure) wherein an impurity density (concentration) gradient is provided in the buffer layer to generate an internal electric field for holes to increase the injection ratio of holes from the anode and enhance the electron storage efficiency, or wherein an impurity density (concentration) gradient is provided in the anode region to generate an internal electric field for electrons and a high-intensity electric field can be applied across the cathode and the anode. The drift buffer structure mentioned above is one that permits the generation of an internal electric field for holes on the basis of an impurity density gradient, but in this specification the term "drift buffer structure" includes also structures of the type that an impurity density (concentration) gradient is provided not only in the buffer layer but also in the anode region adjacent thereto to generate an internal electric field for electrons. The drift buffer structure is further effective when used in combination with a static induction buffer structure.
2. Description of the Prior Art
Heretofore there have been a variety of semiconductor devices of the type having a buffer layer, such as a gate turn-off thyristor of high breakdown voltage, a static induction thyristor, an insulated gate bipolar transistor and an insulated gate static induction thyristor. These semiconductor devices are common in the adoption of the structure wherein an n-type buffer layer is positively interposed between the anode (collector) region and a high-resistivity region of an n-type base layer so that the distribution of the electric field between the gate (base) and the anode (collector) is virtually triangular or trapezoidal to permit uniform application of a high-intensity electric filed to the vicinity of the anode region. This structure allows reduction of the thickness of the high resistivity layer and ease in increasing the breakdown voltage and highly improves the turn-ON characteristic of the device because of the drift motion of carriers in the high resistivity layer under the action of the high-intensity electric field.
The static induction thyristor with an n-type buffer structure has such a construction as disclosed in Japanese Patent Publication No. 31869/84. Moreover, trial manufacture of a 2500V-300A class buried gate static induction thyristor is also reported in "LOW-LOSS HIGH SPEED SWITCHING DEVICE, 2500V-300A STATIC INDUCTION THYRISTOR," PROC. OF THE 16TH ANNUAL IEEE POWER ELECTRONICS SPECIALISTS CONFERENCE (PESC '85).
FIG. 27 is a sectional view schematically showing the internal construction of the above-mentioned buried gate static induction thyristor. In FIG. 27, reference numeral 1 indicates an anode electrode, 2 an anode region, 3 an n-type buffer layer, 4 an n-type buffer short-circuit layer, 5 a high resistivity layer, 6 a gate region, 7 an epitaxial layer, 8 a cathode region, 9 a cathode electrode and 10 a gate electrode. The n-type buffer layer 3 is electrically connected by the n.sup.+ -type region 4 to the p-type anode region 2. The n.sup.+ -type region 4 is formed in the p-type anode region 2 substantially below the gate electrode 10.
The quantity of holes which are injected from the anode side depends on the thickness of the n-type buffer layer 3 and the impurity density (concentration) therein. With too high the impurity density of the n-type buffer layer 3, the quantity of holes injected is small, affecting the turn-ON characteristic and the ON-state voltage. When the impurity density (concentration) of the n-type buffer layer 3 is low, the quantity of holes injected increases but a high-intensity electric field may sometimes enter into the n-type buffer layer 3 and cause a punch through, and hence the breakdown voltage cannot be set so high. This problem could be solved by forming the n-type buffer layer 3 to a relatively large thickness, but such a relatively thick n-type buffer layer with a predetermined impurity density (concentration) is likely to present problems such as an increase in the ON-state voltage, a decrease in the quantity of holes injected and a slow transition to latching up (i.e. a decrease in the turn-ON response speed). It is a general practice in the prior art, therefore, to form the n-type buffer layer relatively thick although it may preferably be thin, besides its impurity density is usually medimum so as to permit the injection of holes in certain quantities although a high impurity density is desirable from the viewpoint of preventing an increase in the breakdown voltage.
Moreover, the n-type buffer layer 3 is interposed, as a region having a predetermined impurity density, between the anode region 2 and the high resistivity layer 5, and if the n-type buffer layer 3 remains electrically floating with respect to the anode region 2, electrons stored in the n-type buffer layer 3 remain therein for a period of time dependent on their lifetime. In this instance, the injection of holes from the anode region 2 occurs and when the lifetime of electrons is long, holes are injected excessively. Hence it is desirable that the n-type buffer layer 3 be electrically shorted to the anode region 2. With an increased short-circuit ratio, the effect of the n-type buffer layer lessens, with the result that no latching up occurs or the quantity of holes to be injected decreases, and in this instance, the turn-ON characteristic of the device is deteriorated even if its turn-OFF and tail characteristics are improved. Since the n-type buffer layer 3 is lamellar, it is also necessary to decrease its resistance in the lateral direction. Moreover, the conventional buffer structure having the base structure has a defect that the ON-state voltage is likely to be high.
There has been well-known a drift transistor which utilizes a structure capable of generating a drift field in the base layer of a bipolar transistor. For semiconductor devices of the type including a thyristor structure with a buffer layer, in particular, however, there has not been proposed a structure which has an impurity density (concentration) gradient set in the buffer layer so that a drift field is generated therein for holes when they are injected from the anode region and/or structure which has a similar impurity density (concentration) gradient set in the anode region to generate a drift field for electrons (which structures will hereinafter be referred to generically as a drift buffer structure).
The drift buffer structure mentioned above is a structure that has an impurity density gradient for generating drift fields for holes and electrons in either one or both of the buffer layer and the anode region adjacent thereto. The reason for which both of the buffer layer and the anode region need to be taken into account is that electrons and holes both contribute to turning ON and OFF the semiconductor devices of the type employing the thyristor structure. It is possible, of course, to use a structure that has the impurity density gradient in only one of the two regions with a view to producing the effect by either electrons or holes alone. It is preferable, however, to implement a structure that permits the generation of drift fields for both of the electrons and holes.